This invention relates to an impregnated cathode for use in electron tubes such as picture tubes, camera tubes, etc.
The impregnated cathode is a promising cathode for electron tubes with a higher performance, and is prepared by impregnating the pores of a porous metal body with an electron emissive material. The porous metal body has been so far made from tungsten, but it is not restricted only to tungsten and can contain refractory metal such as molybdenum, tantalum, etc. The electron emissive material is alkaline earth metal oxides, which comprises barium oxide (BaO) and at least one compound of aluminum oxide (Al.sub.2 O.sub.3), calcium oxide (CaO), magnesium oxide (MgO), etc.
Description will be made hereunder, referring to the porous tungsten body as a typical one and a barium aluminate compound as a typical electron emissive material. The porous tungsten body is prepared from tungsten powder as a starting material by press-shaping the powder, presintering the shaped product in a hydrogen atmosphere at a temperature of 1,000.degree. to 1,200.degree. C., thereby making the handling easier, sintering the presintered product in an unoxidative atmosphere by direct heating by passage of electric current, etc. therethrough and subjecting the sintered product to a machining process, thereby obtaining a cathode of desired form. Direct application of the machining process to the sintered product is difficult to work with, and thus the sintered product is impregnated with copper or plastic to facilitate the machining process, and then is machined to a desired cathode form. Then, the copper or plastic is removed by heated evaporation or by dissolution with an acid.
Porosity of porous tungsten body depends upon the particle size of starting material tungsten powder, press-shaping pressure and sintering conditions in combination. An appropriate porosity is usually about 17 to about 30% by volume on the basis of the sintered tungsten body. Any porosity can be provided by selecting the individual conditions as mentioned above, and thus a porous tungsten body having a desired porosity can be obtained by press-shaping a porous tungsten body having the desired cathode form at first, and then sintering the porous tungsten body. It is rather advantageous to carry out press-shaping at first and then sintering, since the cathodes for picture tubes or camera tubes are usually small in sizes. Furthermore, since this process has no step for impregnating a sintered product with copper or for machining the product into the desired cathode form or for removing the copper, etc. therefrom, the process can be simplified.
Uniform distribution of pores in a porous tungsten body can be obtained by selecting relatively mild sintering conditions under which powder particles can be bonded to one another, because distribution is deteriorated under strict sintering conditions under which the tungsten powders are excessively sintered.
The impregnated cathode can be prepared by placing a barium aluminate compound on the thus prepared porous tungsten body and heating the body in a reduction or unoxidative atmosphere, thereby impregnating the pores of the body with the melted compound, or alternately, the pores of the porous tungsten body can be impregnated with the barium aluminate compound by dipping the body in a molten bath of the compound. While the impregnated cathode is actually used, the tungsten in the body reacts with the barium aluminate compound to form elemental barium, and the elemental barium reaches the surface of the body, i.e. the electron emitting surface, and undergoes surface migration to form a monolayer suitable for electron emission. The thus prepared impregnated cathode is regarded as a promising cathode capable of maintaining a high electron emission for a prolonged time, and its application to small electron tubes such as picture tubes, camera tubes, etc. is not under development. It has a high electron emission, but its operating temperature is as high as 1,050.degree. to 1,200.degree. C., so the evaporation of barium or barium oxide is vigorous, giving a serious influence upon the properties of tubes due to its deposition onto other electrode, or the material of electrode in the oxide-coated cathode or a material of sleeve must be replaced due to the high operating temperature. Furthermore, a heater for the impregnated cathode has such a disadvantage that it fails due to the prolonged use. Thus, investigation has been so far made for electron emissive materials capable of operating at a low temperature, but has not been succeeded yet.
On the other hand, an impregnated cathode, whose electron emissive surface is coated with osmium (Os), osmium (Os)-ruthenium (Ru) alloy, iridium (Ir), osmium (Os)-iridium (Ir) alloy, etc. to a thickness of a few hundred nm, can have a lower operating temperature, for example, by about 150.degree. C. (For example, Japanese Patent Publication No. 2134/72), where the surface is coated by evaporation, sputtering, etc. However, more lowering of operating temperature, for example, by about 150.degree. C. does not solve all the foregoing problems satisfactorily.